The techniques involving the thin-film deposition of semiconductor materials such as hydrogenated amorphous silicon (aSiH), on glass insulating substrates for example, can be used to produce matrices of photosensitive points that can produce an image from a visible or near-visible radiation. To use these matrices in the detection of radiological images, all that is needed is to insert, between the X-radiation and the matrix, a scintillator screen for converting the X-radiation into light radiation in the band of wavelengths to which the photosensitive points are sensitive.
A first defect affects the quality of images acquired by the photosensitive detector. The semiconductor components used in such detectors are not all identical and the photosensitive detector inherently has non-uniformities which are reflected in corrupted areas.
To try to obtain a useful image of optimum quality, a first correction of the image acquired by the detector is performed on the basis of a so-called offset image, known as a “black image”, generally taken and stored at the start of an operating cycle. This offset image is the image obtained when the photosensitive detector is exposed to a signal of zero intensity and corresponds to a kind of background image. The offset image varies as a function of the electrical state of the components of the photosensitive points and of the dispersion of their electrical characteristics. The useful image is the one read when the photosensitive detector has been exposed to a useful signal that corresponds to an exposure to an X-radiation. It encompasses the offset image. The correction consists in performing a subtraction between the useful image and the offset image.
A second correction applied to the image is a gain correction. This correction is generally a multiplying correction and may depend on each photosensitive point. The term “gain image” is then used. This is a matrix of the same size as the photosensitive matrix. The so-called gain matrix comprises, in association with each photosensitive point, a corrective coefficient to be applied to the level measured by the corresponding photosensitive point to obtain a so-called useful image.
The gain image cannot be defined during the normal use of the photosensitive detector. The gain image is defined during a calibration phase which may take a few minutes during which the photosensitive detector is unavailable.
It will be realized that the gain image varies as a function of the temperature of the detector. Sometimes, visible non-uniformities appear in the image and render the image unusable notably in the medical radiology domain. As soon as the temperature varies, typically by around three degrees, it may sometimes be necessary to repeat the calibration phase. To alleviate this problem, temperature-stabilized photosensitive detectors have been produced. To achieve this stability, the detector can be used only after a waiting time of several hours after the detector has been powered up. The temperature of the detector may deviate by more than three degrees from its average temperature. In this case, the image quality may possibly be degraded and it will be necessary to provide for a new detector calibration phase and therefore for the immobilization of said detector. To improve the thermal stability of the detector, coolant circulation has been implemented in the detector. This solution is costly and difficult to implement.
The solutions for stabilizing detector temperature may be adapted to a fixed use, in a room specially dedicated to radiology for example.